The present invention relates generally to an improved composite material. More specifically, the present invention relates to a highly thermally conductive and very high strength composite material that is net-shape moldable and easily moldable or castable by methods such as injection molding.
In the heat sink industries, it has been well known to employ metallic materials for thermal conductivity applications, such as heat dissipation for cooling semiconductor device packages. For these applications, such as thermally conductive heat sinks, the metallic material typically is tooled or machined from bulk metals into the desired configuration. However, such metallic conductive articles are typically very heavy, costly to machine and are susceptible to corrosion. Further, the geometries of machined metallic heat dissipating articles are very limited to the inherent limitations associated with the machining or tooling process. As a result, the requirement of use of metallic materials which are machined into the desired form, place severe limitations on heat sink design particular when it is known that certain geometries, simply by virtue of their design, would realize better efficiency but are not attainable due to the limitations in machining metallic articles. In addition, members that are subject to high stress suffer from the same disadvantages in that they are also difficult to form into the desired configuration while maintaining the desired high strength.
It is widely known in the prior art that improving the over-all geometry of a heat dissipating article can greatly enhance the overall performance of the article even if the material is the same. Similarly, a well designed configuration of an member can greatly improve the strength of the member and reduce its stress and tendency to fracture. Therefore, the need for improved article geometries necessitated an alternative to the machining of bulk metallic materials to provide thermal and electrical transfer. To meet this need, attempts have been made in the prior art to provide molded compositions that include conductive filler and high strength material therein to provide the necessary thermal conductivity and improve structural integrity. The ability to mold a conductive composite enables the design of more complex part geometries to realize improved thermal performance and superior structural integrity of the part.
Prior art compositions are inadequate to address the needs of high thermal conductivity and high structural integrity in the same composition. Typically, a composition must sacrifice structural integrity in favor of higher thermal conductivity or sacrifice thermal conductivity in favor of structural integrity. However, many applications require that a component have both the characteristics of high thermal conductivity and high structural integrity.
The attempts in the prior art included the employment of a polymer base matrix loaded with a granular material, such as boron nitride grains. Also, attempts have been made to provide a polymer base matrix loaded with flake-like filler material. These fillers improve the thermal conductivity of a molded part. These attempts are, indeed, moldable into complex geometries but still do not approach the desired performance levels found in metallic machined parts. In addition, known conductive plastic materials are undesirable because they are typically very expensive to manufacture because they employ very expensive filler materials. Still further, these conductive composite materials must be molded with extreme precision due to concerns of filler alignment during the molding process. Even with precision molding and design, inherent problems of fluid turbulence, collisions with the mold due to complex product geometries make it impossible to position the filler ideally thus causing the composition to perform far less than desirable. Even assuming the proper filler is selected and aligned properly and thermal conductivity is improved, it is common for such a material to have poor structural integrity due to the materials used. For example, high modulus PITCH-based carbon fiber is typically used as a filler to improve thermal conductivity but such a filler is very brittle and is not typically a suitable filler for improving the structural integrity of the composition.
Moreover, the entire matrix of the composition must be satisfactory because heat transfer is a bulk property rather than a direct path property such as the transfer of electricity. Heat is transferred in bulk where the entire volume of the body is employed for the transfer while a direct path is needed to conduct electricity. Therefore, even if a highly conductive narrow conduit is provided through a much lower conductive body, the heat transfer would not be as good as a body which is consistently marginally conductive throughout the entire body. Therefore, consistency of the thermal conductivity of the entire matrix of the composite body is essential for overall high thermal conductivity. Similarly, a consistently strong matrix of composite material is highly desirable to improve the overall structural integrity of the finished molded composite part.
In view of the foregoing, there is a demand for a composite material that is highly thermally conductive and of high structural integrity. In addition, there is a demand for a composite material that can be molded or cast into complex product geometries to enhance thermal conductivity and structural integrity. There is also a demand for such a moldable article that exhibits thermal conductivity and structural integrity as close as possible to or in excess of purely metallic conductive materials while being lightweight and relatively low in cost to manufacture.
The present invention preserves the advantages of prior art thermally conductive and structurally enhanced plastic compositions. In addition, it provides new advantages not found in currently available compositions and overcomes many disadvantages of such currently available compositions.
The invention is generally directed to the novel and unique highly thermally conductive plastic composite material with particular application in heat sink applications where heat must be moved from one region to another to avoid device failure. According the application at hand, the highly thermally conductive part made from the composition of the present invention may also be required to exhibit high structural integrity as well. For example, an outer case for a mobile phone requires high structural integrity to prevent damage to the delicate electrical component parts housed therein. However, the outer case may also be needed to help dissipate heat generated by the same component parts.
The composite material of the present invention enables a highly thermally and high structural integrity composite material to be manufactured at relatively low cost to meet both needs at the same time in a single composite material. The conductive molding composition of the present invention has a thermal conductivity of approximately 4 W/mxc2x0 K or more and a tensile strength of at least approximately 15 ksi and a flexural strength of at least approximately 20 ksi and a flexural modulus of at least approximately 2 Msi.
The conductive composition of the present invention includes a polymer base matrix of, by volume, between 30 and 70 percent. The base matrix is preferably polycarbonate material but may also be liquid crystal polymer material. A first thermally conductive filler of high modulus PITCH-based carbon material, by volume, between 15 and 47 percent is provided in the composition that has a relatively high aspect ratio of at least 10:1. A second filler of PAN-based carbon filler is also provided to greatly improve the strength of the composition and is provided, by volume, between 10 and 35 percent and also has a high aspect ratio of at least 10:1. The total fiber content of the composition is preferably in the range of 30 to 70 percent by volume. The ratio of the high modulus PITCH-based carbon fiber to the PAN-based carbon fiber is preferably in the range of about 1:1 to 2:1. Optionally, the mixture also includes a third filler to improve the thermal conductivity of the composition which is provided, by volume, between 1 and 10 percent that has a relatively low aspect ratio of 5:1 or less.
During the molding process of the composition of the present invention, the mixture is introduced into a mold cavity and flows into the various part geometries. The high aspect ratio first filler and high aspect ratio second filler generally align with the flow of the mixture in the mold. The third filler, of a low aspect ratio, may be optionally added in the mixture to fill the voids between the first and second high aspect ratio fillers in the mixture. As a result, the number of interfaces and base matrix thickness between filler members is greatly reduced thus resulting in thermal conductivity and performance superior to that found in prior art conductive composite materials while still providing a high strength molding composition.
It is therefore an object of the present invention to provide a highly thermally conductive composite material that also has high structural integrity.
It is an object of the present invention to provide a highly thermally conductive composite material of high structural integrity that is moldable, such as by injection molding.
It is a further object of the present invention to provide a low cost conductive composite material.
Another object of the present invention is to provide a conductive composite material that enables the molding of complex part geometries to enhance thermal conductivity and structural integrity.
It is a further object of the present invention to provide a highly thermally conductive composite material of high structural integrity that is significantly lighter in weight than metallic materials.
It is yet a further object of the present invention to provide a highly thermally conductive composite material of high structural integrity that has a thermal and strength close to pure or composite metallic materials.